1. Field of the Invention
The present disclosure generally relates to image processing and, more particularly, to an image processing apparatus, an image processing method, and an image recording apparatus.
2. Description of the Related Art
There has been available a recording scheme which causes a recording head having a plurality of arrays of ejection ports for ejecting ink to eject ink while scanning across a recording medium to perform a scanning and recording operation to form an image on the recording medium.
In such a recording scheme, one of the generally applicable techniques for quantization of multi-valued data of an image is the so-called dithering technique. In the dithering technique, a dither pattern in which threshold values are set for individual pixels to determine ejection or non-ejection of ink to each of a plurality of pixel areas is used for quantization. U.S. Patent Application Publication No. 2007/0097164 discloses that a dither pattern in which threshold values are set so that respective threshold values for a plurality of pixels have a small number of low-frequency components to reduce the graininess of an image to be obtained is used for quantization.
Meanwhile, various types of ink and recording media have been used in recent years for the recording method described above. One known combination of ink and recording medium is the use of ink including a pigment in combination with a recording medium having low ink permeability. The use of such ink and recording medium to record an image may cause the occurrence of thin-film interference since the ink is fixed to a surface of the recording medium, potentially resulting in a change in the color tint of an image to be obtained. It is known that thin-film interference occurs more markedly when ink is ejected a relatively smaller amount and when an ink layer formed on the recording medium is thinner. U.S. Patent Application Publication No. 2013/0300788 discloses that when ink is ejected a small amount, the amount of image-quality improving liquid to be applied relatively increases to increase surface irregularities of an ink layer to suppress the occurrence of thin-film interference.
However, recording using the method disclosed in U.S. Patent Application Publication No. 2013/0300788 may raise an issue of insufficient suppression of the occurrence of thin-film interference.
Such an issue will be described in detail hereinbelow.
FIG. 1 is a diagram depicting a change in color tint due to thin-film interference.
Here, consideration will be given to the case where, for simplicity, light (incident light) 71 enters an ink layer (thin film) 70 having a dot height d, which is formed on a recording medium 3, at an angle of incidence of 45°, and reflected light produced by the specular reflection of the incident light 71 is observed. The reflected light is obtained by combining a reflected light beam 73 produced by the specular reflection of the incident light 71 at a surface of the recording medium 3 after penetrating the ink layer 70, and a reflected light beam 72 produced by the specular reflection of the incident light 71 at a surface of the ink layer 70. There is an optical path difference between the reflected light beam 72 and the reflected light beam 73 over a period from the entry of the incident light 71 to the observation of the reflected light. It is known that the value of the optical path difference can be approximated to be about 2d when the dot height d of the ink layer 70 is sufficiently low.
From the discussion described above, it is understood that when the dot height d of the ink layer 70 is low (the ink layer 70 is thin), the optical path difference between the reflected light beam 72 and the reflected light beam 73 is short. This causes interference, such as strengthening or weakening, between the reflected light beam 72 and the reflected light beam 73. Specifically, if the optical path difference, which is given by 2d, is equal to an integer (m) multiple (2d=mλ) of the wavelength λ of the incident light 71, the reflected light beam 72 and the reflected light beam 73 strengthen each other. If the optical path difference, which is given by 2d, is equal to the sum of an integer multiple of the wavelength λ of the incident light 71 and one-half the wavelength λ (2d=(m+½)λ), the reflected light beam 72 and the reflected light beam 73 weaken each other. When interference of only either strengthening or weakening occurs, the intensity of the reflected light to be observed is different from the intensity of the incident light 71. Consequently, the color tint of an image changes.
As described above, thin-film interference occurs markedly for an ink layer having a low dot height. An examination performed by the inventors reveals that if different types of ink are used, the dot heights might differ even though the amounts of ejected ink are the same. The examination also reveals that the degree of occurrence of thin-film interference changes depending on the type of ink to be used.
FIGS. 2A to 2C are diagrams depicting the degrees of occurrence of thin-film interference when different types of ink are used. FIG. 2A is a diagram illustrating the degree of occurrence of thin-film interference when ink having a relatively low dot height when applied to a recording medium is used. FIG. 2B is a diagram illustrating the degree of occurrence of thin-film interference when ink having a relatively high dot height when applied to a recording medium is used. FIG. 2C is a diagram illustrating the degree of occurrence of thin-film interference when ink having a relatively low dot height and ink having a relatively high dot height are both used. Note that, in the illustrations of FIGS. 2A, 2B, and 2C, the amounts of ink ejected is the same.
As described above, thin-film interference occurs markedly when ink having a relatively low dot height (d1) is used. Since an ink layer 74 is formed to be comparatively flat, interference occurs in substantially the same form regardless of where incident light enters on the ink layer 74. Thus, in the ink layer 74 illustrated in FIG. 2A, the color tint of an image to be observed markedly changes.
In contrast, when ink having a relatively high dot height (d2) is used to form an ink layer 75, as illustrated in FIG. 2B, the optical path difference between reflected light beams is also relatively long. Accordingly, a plurality of kinds of interference, such as strengthening and weakening, occur between the reflected light beams. As a result, a change in color tint that is biased in a specific direction does not occur. If the ink layer 75 has a high dot height to some extent, the ink layer 75 will form a projection, as illustrated in FIG. 2B. Thus, the dot height is equal to d2 around the center of the ink layer 75, whereas the dot height is equal to a value lower than d2 around the ends of the ink layer 75. In the manner described above, the optical path difference between reflected light beams differs depending on the position of incidence of incident light, and thus various kinds of interference occur depending on the position. Accordingly, when ink having a relatively high dot height is used, reflected light beams interfere with one another in a multiplex manner, resulting in the change in the color tint of an image to be observed becoming less noticeable.
As illustrated in FIG. 2C, when both ink having a relatively low dot height and ink having a relatively high dot height are used, a change in color tint due to thin-film interference occurs on the ink layer 74 formed by the ink having a relatively low dot height in a manner similar to that in the image illustrated in FIG. 2A. On the ink layer 75 formed by the ink having a relatively high dot height, in contrast, reflected light beams interfere with one another in a multiplex manner, as in the image illustrated in FIG. 2B. The multiplex interference of reflected light beams on the ink layer 75 will suppress the effect of thin-film interference on the ink layer 74, resulting in a reduced change in the color tint of the entire image, compared to the image illustrated in FIG. 2A.
As described above, the examination performed by the inventors reveals that the change in color tint due to thin-film interference may differ depending on the dot height of ink to be used, even if the same amount of ink is ejected per unit area on a recording medium. Specifically, a change in color tint caused by thin-film interference is more likely to occur on an image formed by ink whose dot height is low when ink having a high dot height is ejected a relatively small amount than on an image formed by ink whose dot height is low when ink having a high dot height is ejected a relatively large amount.